Rheological Controls on Creeping Landslides Within the Franciscan Mélange: Insights From Laboratory Experiments

Landslides and tectonic faults exhibit a spectrum of slip behavior, from steady frictional creep to catastrophic failure. However, the mechanisms controlling slow slip, and slip speed more generally, remains unresolved. Here we report on laboratory shearing experiments on natural samples from the localized shear plane of a well-instrumented, active, slow landslide within the Franciscan mélange in central California. The shear plane material exhibits velocity strengthening frictional behavior over a range of slip velocities (0.001–30 µm/s), consistent with velocity strengthening friction previously inferred from field observations of landslide motion and pore pressure. Our results suggest that laboratory measurements of frictional rheology may provide a pathway to predict landslide behavior in the Franciscan as well as other lithologies. Fundamentally, our experimental data suggest that slow slip transients may arise in velocity strengthening material across a range of geologic and tectonic settings through the modulation of effective stress.